We are using a commercial vendor of video processor boasrds (NVidia) which are both substantially less expensive than the previous generation of custom volume rendering boards as well as haveing a greater potential for growth in performance. The commercial vendor also provides software (CUDA) for utilizing the boards for Monte Carlo calculations, which further supports our research objectives. We have transitioned from 3-dimensional image fusion to exploring 4-dimensional radiotherapy imaging. Our research has established a fundamental relationship between the temporal motion of the 3-dimensional external torso volume and those of internal organs, especially the lungs. Our research has developed a volumetric methodology for image tracking using external torso volume change for which a patent has been applied for. The Electron-Gamma Shower (EGS-4)-based Monte Carlo Dose Calculation Engine (DCE) has been fully implemented in both a LINUX and Windows environment. In the Windows environment, the DCE has been integrated into a full featured treatment planning system. Work is now centered on the development of phase-space source models. Currently, this system is used to investigate small field stereotactic radiosurgery. Due to the reduced field size, edge effects become important and the size of detectors used to access the radiation output affect the measurement results. Monte Carlo simulation of these output measurements greatly assisted in the selection of the detector system that is most suitable for these measurements. The Monte Carlo algorithm provided good agreement with experimental measurements down to applicators as small as 5mm. In a related project, we have adapted both algebraic and Monte Carlo DCEs to predict organ doses received from diagnostic CT scans. The characterization of the x-ray beam from a GE CT Scanner used for clinical scanning at Children's National Medical Center is complete. We have also completed absolute dosimetry measurements linking the standard diagnostic measurement of Computer Tomography Dose Index (CTDI) to actual absorbed dose. We are awaiting the arrival of a new postdoctoral research associate to replace our former postdoc, who left unexpectedly to accept a position within the Food and Drug Administration (FDA). The candidate selected for this position has experience in image guided radiotherapy to expand on these projects, but will not be able to start until she completes her degree requirements in the Fall of 2009. The next step for image tracking will be to provide an interface to an accelerator to control couch morion in order to compensate for patient motion or, alternatively, to alter the sequence of multi-leaf collimator leaves. The MOnte Carlo CT project needs to be expanded to multi-slice CT scanners and the normalization methodology needs to be shifted away from CTDI to a Bragg-Gray cavity approach, similiar to that employed traditionally by radiotherapy physicists.We are using a commercial vendor of video processor boasrds (NVidia) which are both substantially less expensive than the previous generation of custom volume rendering boards as well as haveing a greater potential for growth in performance. The commercial vendor also provides software (CUDA) for utilizing the boards for Monte Carlo calculations, which further supports our research objectives. We have transitioned from 3-dimensional image fusion to exploring 4-dimensional radiotherapy imaging. Our research has established a fundamental relationship between the temporal motion of the 3-dimensional external torso volume and those of internal organs, especially the lungs. Our research has developed a volumetric methodology for image tracking using external torso volume change for which a patent has been applied for. The Electron-Gamma Shower (EGS-4)-based Monte Carlo Dose Calculation Engine (DCE) has been fully implemented in both a LINUX and Windows environment. In the Windows environment, the DCE has been integrated into a full featured treatment planning system. Work is now centered on the development of phase-space source models. Currently, this system is used to investigate small field stereotactic radiosurgery. Due to the reduced field size, edge effects become important and the size of detectors used to access the radiation output affect the measurement results. Monte Carlo simulation of these output measurements greatly assisted in the selection of the detector system that is most suitable for these measurements. The Monte Carlo algorithm provided good agreement with experimental measurements down to applicators as small as 5mm. In a related project, we have adapted both algebraic and Monte Carlo DCEs to predict organ doses received from diagnostic CT scans. The characterization of the x-ray beam from a GE CT Scanner used for clinical scanning at Children's National Medical Center is complete. We have also completed absolute dosimetry measurements linking the standard diagnostic measurement of Computer Tomography Dose Index (CTDI) to actual absorbed dose. We are awaiting the arrival of a new postdoctoral research associate to replace our former postdoc, who left unexpectedly to accept a position within the Food and Drug Administration (FDA). The candidate selected for this position has experience in image guided radiotherapy to expand on these projects, but will not be able to start until she completes her degree requirements in the Fall of 2009. The next step for image tracking will be to provide an interface to an accelerator to control couch morion in order to compensate for patient motion or, alternatively, to alter the sequence of multi-leaf collimator leaves. The MOnte Carlo CT project needs to be expanded to multi-slice CT scanners and the normalization methodology needs to be shifted away from CTDI to a Bragg-Gray cavity approach, similiar to that employed traditionally by radiotherapy physicists.